The wide range of interesting and useful properties exhibited by ceramic films makes them technically important. The ceramic films can be deposited or coated on objects to prevent the coated objects from corrosion or attrition. On the other hand, the ceramic films exhibiting specific electric properties constitute key components in a wide range of electronic and optical devices. The electronic devices using ceramic films include ferroelectric memories, piezoelectric transducers, capacitors, dielectric resonators, gas-sensors, pyroelectric sensors, actuators, and transducers. Particularly, the ferroelectric ceramic thin films have increasingly attracted great interest for the use in nonvolatile random access memories (NVRAM) due to their large reversible spontaneous polarization, and for the use in dynamic access memories (DRAM) owing to their high dielectric constant. As for the optical devices, ceramic films have been applied in non-liner optical devices, electro-optic switching, electro-optic displays, and reflective/antireflective coating.
To prepare ceramic films for different applications, various types of processing methods have been developed. These methods can be classified into two categories: namely, chemical and physical processes. For chemical processes, chemical vapor deposition, spin coating, dipping, and sol-gel processing have been developed. As for the physical processes, evaporation, ion-beam deposition, molecular beam epitaxy, radio-frequency and DC sputtering, and laser ablation have been widely investigated. In general, the as-grown ceramic films deposited on substrates at low temperatures are amorphous or partially crystallized. In order to completely crystallize the as-prepared films, either the high temperatures on substrates during the deposition processes or the post-annealing process at high temperatures for deposited films is required. Usually, the crystallization process of ceramic films demands fairly high temperatures. For example, the crystallization temperature for Pb(Zr, Ti)O.sub.3 and SrTiO.sub.3 is 600.degree. C. at least, and that for SrBi.sub.2 Ta.sub.2 O.sub.9 is above 650.degree. C. The high-temperature heating often results in interference in the film-substrate interface, and causes difficulties in integrating the films with substrates. The most common substrates for ferroelectric ceramic films are silicon substrates owing to their wide applications in ULSI technology. During the high-temperature annealing processes, the silicon will be oxidized to form silicon dioxide layers, bringing difficulties in integrating the ceramic films with silicon monolithic circuits. The high-temperature annealing also significantly enhances the diffusion of the species into ceramic films as well as substrates, thereby rendering the interdiffusion and interaction between films and substrates. The undesirable reactions occurring in the film-substrate interface result in the deviation of the composition in ceramic films and doping of foreign atoms in silicon circuits which cause serious problems in varying the electrical properties of ceramic films as well as silicon circuits. The high-temperature annealing also enhances the grain growth rate on films and results in coarsening of grains and increases the roughness of films. The rough morphology of films increases the difficulties in the subsequent processes of etching and patterning. As a result, the high temperature processing for preparing ceramic films is not suitable to be integrated into the silicon processing technologies, due to the temperature limitations as to the stability of the underlying silicon wafers and structures and properties of ceramic films.
In order to overcome the drawbacks of high-temperature processing for crystallizing as-deposited ceramic films, new crystallization processes such as hydrothermal processing, hydrothermal electrochemical processing, laser annealing, ion-beam bombardment, electron-irradiation, and plasma processing have been developed recently. In hydrothermal processing and hydrothermal electrochemical processing, substrates are immersed into aqueous solutions containing constituent species and are reacted with solutions to form the desired compounds on the substrate surface (see, for example, Yoshimura et al., Solid State Ionics, Vol. 98 (3/4), p. 197 (1997)). The hydrothermal processes prepare various ceramic films at relatively low temperatures ranging from 100.degree. C. to 300.degree. C. However, in these processes, the substrates are immersed in reactive solutions and directly contact with reactive solutions. Since the reactive solutions usually contain high concentration of acid or alkaline reactants, undesirable corrosion will easily occur on the substrates. As for USLI technology, the silicon substrates usually have different patterns or metal lines deposited on the surface of substrates. Due to the corrosion problems, the hydrothermal processing is not suitable for ULSI technology.
Laser-induced crystallization processes have been investigated recently. Lu et al., Appl. Phys. Lett. Vol. 65 (16), p. 2015 (1994), used rf-magnetron sputtering to prepare PbZr.sub.0.44 Ti.sub.0.56 O.sub.3 (PZT) amorphous ceramic films on glass substrates, then laser scanned the resultant films. The output power of the laser-scan was 2.5 W, beam spot diameter was 90 .mu.m, and the scanning rate was 4.5 cm/s. Under these conditions, the amorphous PZT films can be transformed into crystallized state at room temperature. Varshney, U.S. Pat. No. 5,626,670, also used similar laser techniques to enhance the crystallization of PZT films prepared by the spin-on sol-gel process.
On the other hand, Yu et. al., Phys. Rev. Vol. 52 (24), p. 17518 (1995), investigated the crystallization of alumina films induced by ion-beam bombardment. Amorphous alumina films were coated on single crystal alumina, and then bombarded with argon or oxygen ions at temperatures ranging from 400.degree. C. to 600.degree. C. This study indicates that the ion-beam bombardment effectively induces the amorphous-to-.gamma. phase transformation of alumina. Yu et. al., Mater. Chem. and Phys. Vol. 46 (2/3), p.161 (1996), also employed electron irradiation to facilitate the crystallization of amorphous MgAl.sub.2 O.sub.4 films. Single crystal MgAl.sub.2 O.sub.4 substrates were coated with amorphous MgAl.sub.2 O.sub.4 films by Xe ion irradiation. After the coated films were subjected to electron irradiation at 300 keV at room temperature, the crystallized MgAl.sub.2 O.sub.4 films were obtained. Carl et al, U.S. Pat. No. 5,468,687, utilized the ozone enhanced plasma to enhance the crystallization of Ta.sub.2 O.sub.5 films prepared by chemical vapor deposition, and reduced the annealing temperature to as low as 400.degree. C.
Although the laser annealing, ion-beam bombardment, and electron-irradiation techniques can successfully induce the crystallization of amorphous ceramic films at relatively low temperatures, the small beams of laser, ion, and electron beams pose significant concerns when the above technologies are applied to mass produce crystallized ceramic films having large surface areas. Using the scanning technique during the irradiation processes can enlarge the area of crystallized ceramic films; however, the low scanning rate limits the throughput of ceramic films, and the possibility of mass production. On the other hand, the high excitation energy sources including laser, ion, electron beams, and plasma will damage ceramic films and create defects in films, thereby deteriorating the electrical properties of prepared films. Thus, in order to overcome the disadvantages encountered in the prior methods, a process that can crystallize as-deposited or as-coated ceramic films at relatively low temperatures and is practicable for mass production, would be highly desirable.